<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      ul li {
        cursor: pointer;
        background-color: aqua;
      }
      .active{
        background-color: red;
      }
    </style>
  </head>

  <body>
    <button>按钮1</button>
    <button>按钮2</button>
    <button>按钮3</button>
    <button>按钮4</button>
    <script>
     let btn=  document.querySelectorAll("button")
 
      for(let i=0; i<btn.length; i++){
        btn[i].onclick=function(){
          for(let j=0; j<btn.length; j++){
            btn[j].classList.remove("active")
          }
         this.classList.add("active")
        }
      }
    </script>
  </body>
</html>
